1. Field of the Invention
The present invention relates to fuzzy controllers used in fuzzy control.
2. Description of the Prior Art
A fuzzy controller is used for inputting one or a plurality of signals representing controlled variables obtained from a system to be controlled, implementing fuzzy inferences using predetermined membership functions in accordance with a predetermined rule, and outputting a non-fuzzy value derived from the results of the fuzzy inferences as a manipulated variable relative to the system to be controlled.
The fuzzy inferences are implemented in accordance with a Modus Ponens, which in many cases, is expressed by an IF-THEN rule (control rule). For simplicity, when there are two types of input variables, this rule is expressed as follows:
"If x is A and y is B, then z is C."
or
"If x=A and y=B, then z=C."
Here x and y are input variables, z is an output variable, and A, B and C are fuzzy sets or membership functions. If x=A and y=B" is referred to as an "antecedent", and "then z=C" is referred to as a "consequent". A and B are also referred to as membership functions relative to inputs (input variables), and C is also referred to as a membership function relative to an output (output variable).
In the fuzzy controller, a plurality of rules are prepared. In addition, a plurality of types of membership functions are set. The membership functions frequently used include PL (Positive Large), PM (Positive Medium), PS (Positive Small), ZR (almost Zero), NS (Negative Small), NM (Negative Medium), and NL (Negative Large). The symbols PL, PM, ZR, NS, NM and NL are membership functions expressed by linguistic information, which are also referred to as fuzzy labels.
Input and output ranges of the conventional fuzzy controller are fixed. On the contrary, input and output ranges of a sensor and an actuator respectively connected to input and output terminals of the fuzzy controller are various, for example, 0.about.5 V, 0.about.10 V, 1.about.5 V, 5 V, and 10 V. Accordingly, the conventional fuzzy controller has the disadvantage in that a range converter is required for each sensor and for each actuator.
Furthermore, in the conventional fuzzy controller, a controlled variable (an observed value) obtained from the system to be controlled is used as an input value without any modification and a fuzzy inference value (a value converted into a non-fuzzy quantity) is used as an output value without any modification. Accordingly, the conventional fuzzy controller has some disadvantages. For example, the system to be controlled is slow in response at the time of controlling of the start and the stop thereof if the membership functions relative to the inputs and output remain fixed. In addition, the offset of the system to be controlled from a target and the vibration thereof are liable to be produced in the control in which a target value is fixed. The disadvantages occur due to the fact that the scale of the membership functions is fixed. In other words, the relationship between the inputs and output and the membership functions corresponding thereto is fixed, thereby making it impossible to ensure considerable input and output gains.
Additionally, in the conventional fuzzy controller, the range in which the input variables vary is generally covered by 7 to 11 membership functions. When such a small number of membership functions are set, resolution is decreased. Therefore, even if a high-precision input value is obtained, it is difficult to exercise precise control corresponding to the precision. In particular, it is desirable that the output is controlled with higher precision in the steady state where the inputs are stable. However, this goal can not be satisfied.
The fuzzy controller the input and output dynamic ranges of which are fixed has the disadvantage of being unable to deal with a case where it is desired to execute adaptive control by varying the input and output dynamic ranges in real time.
In the fuzzy controller, the input variable in the antecedent are generally output values of the sensor, and the output variable in the consequent is, for example, a quantity of displacement of the actuator. The offset of the output value and the gain differ for each sensor and for each actuator. Accordingly, the fuzzy controller has the disadvantage in that data representing the membership functions must be adjusted, the work thereof being laborious.